When Mexican street food chain Wahaca was hit with a norovirus outbreak, co-founder Thomasina Miers was determined to come back

Thomasina Miers blinks back tears as she tells me a “horrendous” tale from Wahaca’s recent norovirus outbreak. “A family had their wedding party in the Edinburgh branch,” says the 40-year-old, who co-founded the Mexican food chain 10 years ago. “A bridesmaid was the first to get sick, as the bride was getting dressed.” She shudders. “And it culminated on the dancefloor with the groom spewing.”

The crisis last November left more than 300 customers ill and temporarily closed its restaurants. Miers calls it “overly humbling”. Her business partner, Mark Selby, apologised in person to the wedding party, while Miers hand-signed letters with seeds sewn into the paper that recipients could plant so that “something good would come”.

She is proud that Wahaca tackled it head-on. “We threw everything we had at it. It cost us an absolute fortune but we were determined not to jeopardise anyone we didn’t have to.” Takings plunged in the aftermath — especially at sites like Canary Wharf, where firms sent messages to staff: “Don’t go near Wahaca” — but are now recovering. Miers has just returned from maternity leave following the birth of her third child, seven-month-old Isadora. Not that she is referred to like that. “My husband calls her ‘wingnut’ because she has sticky-out ears.” She smiles, guiltily. “OK, we all call her that. We’ll have to stop before she works out why.”

Coming back after maternity leave, Miers feels, is “always traumatic”. But there was no chance of her splitting leave with investment banker husband Mark Williams. “Paternity leave is laughable. It’s way too expensive for him to take time off. He took a week’s holiday to help, and that was it.”

Isadora has two older sisters, Tatyana and Ottilie. “I was pleased when I got girls, though I quite wanted a boy. There should be more strong men in the world — I’ve got strong women in my life already.”

Miers has made a habit of doing “power maternity leave”. After she had Ottilie, she reared pigs. This time she “nearly killed” herself writing a new recipe book, Home Cook. “Halfway through, I wondered, ‘Why am I doing this?’ If I was a youngster, I’d think ‘Screw the cookbooks and do a YouTube channel’.” The book is about healthy eating but definitely not “clean eating”. “What we hear about healthy eating now is buying inaccessible, expensive ingredients from far-flung places,” she says, alluding to the Deliciously Ella spiraliser army. “Healthy eating is about cooking with unprocessed food and buying locally and in season. If you can eat more vegetables and less meat, that’s it. It’s not about not eating cream.”

This attitude stems from her past. She had “massive food issues” in her teens and was “always on a diet”. Now she thinks it is vital to remember that “food is about pleasure and saying ‘yes’”. Miers is a rare breed: a female entrepreneur. She wants to encourage more women to launch businesses. “We’re natural entrepreneurs. We are creative and adaptable, and we’re much tougher [than men]. Man flu leaves men bedridden. Women just get on with it.” Miers confesses that both Marks — husband and business partner — describe her as a “total ball-breaker”. “It’s slightly worrying! I’m totally not, but this does toughen you up. I couldn’t say boo to a goose a decade ago.” She advises young women to be more brazen, especially when it comes to money. “Having fought to get a pay rise once, I spoke to lots of girlfriends. Bright women. They hadn’t had a rise for five years but hadn’t felt they were worth it. Men aren’t like that. They feel entitled and have a false sense of their own worth.”

She thinks you have to be prepared to quit when you ask for a raise. “I once got told, ‘Tommi, you act like this is just a job. I thought you loved going to work’. You’d never say that to a man! It’s such a joke.”

Childcare, she believes, is the biggest barrier to female career advancement. “It’s prohibitively expensive. I know so many women who would love to jump back into work who can’t afford it.” When I suggest that childcare could be entirely tax deductible, she leaps on the idea: “That’s a good campaign!”

Miers seems to collect causes. Her main one is the environment, a passion she inherited. “My mother bathes in the water my father showers in.” As such, she confesses that having a third child felt “self-indulgent... There are too many people in the world. Look at David Attenborough and Planet Earth. But my brother has no children, my sister only has one and my sister-in-law has none, so we took their quota. That’s how we rationalise it.” She is also a determined advocate for the restaurant industry, calling for the Government to stop “crippling” it. “Rents and service charges are extortionate. The living wage is a difficult one because people have to afford to live but the problem is that successive governments failed to build housing as our population increased.” She is, she laughs, “getting more socialist” every day. “All these luxury flats! London is going to get hollowed out. It’s positive having an influx of creative people who want to work but you have to build houses for them.”

The noro crisis and Wahaca’s 10-year anniversary have also given Miers a chance to scrutinise the business — including the menu. “I have been trying to shrink it for ages but the team always says ‘No, I love that’. Now I’ve got carte blanche to tuck it in.” The other impetus for change has been new Mexican rivals such as Soho’s Corazon and El Pastor in Borough. “We’re finally getting some decent competition!”

Although Wahaca has been at the forefront of trends — no bookings, sharing plates, mezcal — Miers wants to keep adapting. She regularly visits Mexico for ideas and is heading there in April. So what does she make of the US President’s plans to build a border wall? “I’m baffled by Trump. I can’t believe the Democrats didn’t put a suitable person to stand against him. The Mexicans prop up the entire American industry — not just restaurants, everything. He’s living in la-la land. When I go to LA they understand my Spanish more than they do my English.”

She stops, thoughtful. “Maybe it will actually help Mexico, because [the US] might stop shipping so many arms there and buying so many drugs. They’re outsourcing the drugs war to Mexico.” We turn to the other big upset of 2016, the EU referendum result. Miers says Wahaca “tends to do well in crises”, having opened its first site just before the 2008 recession. “Brexit is weird because it’s hard to get Mexican chefs over because of visas, and Brexit might help that. But we’ve got a staff party tonight — we have 1,200 staff from every nation — they’re great kids, and the idea that you’d stop people coming over...” She sounds exasperated. “Lots of Brits don’t want to do [restaurant] jobs.” This is, she adds, changing slowly. “When we opened, graduates never went into this. You wanted to be a lawyer or whatever. But since the recession, and since food has become such a thing, more young, bright people see the food industry as a viable career.”

Miers was a forerunner. Having studied at St Paul’s, she initially shied away from entering the food industry: “Entrepreneurship used to be seen as “dodgy — all that smooth-talking.” It was her mentor, Clarissa Dickson Wright, who told Miers to “just do it”: “Why did I waste from 18 to 26 examining my belly button?” she laughs.

I ask if she feels overwhelmed juggling a business with so much else. “I’m trying to say no to more things,” she replies, before telling me about yet another project she’s involved in, trying to reduce antibiotic use in livestock. She smiles her wide, gap-toothed grin. “I get carried away.”